如果我有这三个表(只是一个例子来学习UNION,这些不是真正的表)：
表格及其列：
Customer: id | name | order_status Order_Web: id | customer_id | order_filled Order: id | customer_id | order_filled
我想在Order_Web表或使用Union的客户的Order表中填写订单时更新Customer表中的order_status：
UPDATE c
SET c.order_status = 1
FROM Customer AS c
INNER JOIN Order_Web As ow
ON c.id = ow.customer_id
WHERE ow.order_filled = 1
UPDATE c
SET c.order_status = 1
FROM Customer AS c
INNER JOIN Order As o
ON c.id = o.customer_id
WHERE o.order_filled = 1
如何在order_web和订单上使用Union来组合这两个更新？
解决方法
你不需要UINION – 用一对外连接替换内连接应该这样做：
UPDATE c SET c.order_status = 1 FROM Customer AS c LEFT OUTER JOIN Order_Web As ow ON c.id = ow.customer_id LEFT OUTER JOIN Order As o ON c.id = o.customer_id WHERE ow.order_filled = 1 OR o.order_filled = 1
您还可以使用WHERE EXISTS,如下所示：
UPDATE c
SET c.order_status = 1
FROM Customer AS c
WHERE EXISTS (
SELECT 1 FROM Order_Web As ow WHERE c.id = ow.customer_id AND ow.order_filled = 1
) OR EXISTS (
SELECT 1 FROM Order As o WHERE c.id = o.customer_id AND o.order_filled = 1
)
如果必须使用UNION,可以按如下方式执行：
UPDATE c
SET c.order_status = 1
FROM Customer AS c
WHERE c.id in (
SELECT ow.id FROM Order_Web As ow WHERE ow.order_filled = 1
UNION
SELECT o.id FROM Order As o WHERE o.order_filled = 1
)
或者与JOIN相同的人：
UPDATE c
SET c.order_status = 1
FROM Customer AS c
JOIN (
SELECT ow.id AS id FROM Order_Web As ow WHERE ow.order_filled = 1
UNION
SELECT o.id AS id FROM Order As o WHERE o.order_filled = 1
) AS ids ON ids.id = c.id
